1. Technical Field
This invention relates in general to networks and, more particularly, to metropolitan networks.
2. Description of the Related Art
FIG. 1 illustrates a basic block diagram of a metropolitan access network 10 using a DWDM (Dense Wavelength Division Multiplexing) ring 12 and a MAC (medium access control) protocol. A plurality of stations 14 are coupled to the DWDM ring 12. The stations 14 may be coupled to other networks, such as local area networks and wide area networks, as well. In the illustrated embodiment, Station1 and Station2 are coupled to LANs (local area networks) 16, and Station3 is coupled to a WAN (wide area network) 18. Station4 is a stand-alone computer. Various station types may be coupled to the ring, such as stand-alone computers, servers, packet ADMs (Add/Drop Multiplexer), bridges, gateways, IP routers and optical cross-connects.
In operation, metropolitan access networks (also referred to as MANs) are used to connect stations in an area spanning up to 100 kilometers. Voice, video, and data may be carried over the MAN 10. Typically, less than several hundred stations 14 are coupled to the ring 12 in a metropolitan access network 10. In the arrangement shown, information is communicated on ring 12 over multiple channels 20, each channel 20 having an associated unique wavelength (λ), as shown in FIG. 2. One of the channels 20 is designated as the control channel 22. The remaining n channels are data channels 24 which can carry either voice or data information.
Communications between stations 14 is provided through reservations of the channels by a station 14 that wishes to initiate a communication with one or more other stations 14. For example, Station1 could communicate to Station2 by reserving the λ3 data channel. Once reserved, only Station1 and Station2 could use this channel; the other stations 14 would be blocked out from using λ3, but could use any other available channel. Station1 could also multicast data to multiple stations 14 using a single data channel 24. Further, a single station 14 could communicate independently with multiple stations over separate channels. For example, Station1 could be communicating with Station2 over channel λ1 and could be communicating with Station3 over channel λ2 at the same time, if such an interface is implemented at the stations.
To allocate the data channels 24 between the various stations 14, the control channel 22 continuously circulates tokens 26 between all the stations 14 on the ring 12, as shown in FIG. 3. Each token 26 has a one-to-one relationship with a data channel 24; hence there are n tokens 26 used to represent the n data channels 24. When a first station 14 needs to communicate with a second station 14 on the ring 12, it monitors the tokens 26 passed over the control channel 22. Each token will have a field indicating the availability of the channel. When the first station 14 receives a token indicating its availability, the first station modifies the token 26 to indicate the desired communication (in this case, a point-to-point communication to the second station) and changes the token to indicate that the associated data channel 24 is currently occupied. When the modified token 26 is received by the second station 14, an acknowledgement is sent to the first station 14 and the selected channel 24 is set up for the communication between the two stations 14. When the communication is finished, the first station 14 releases the communication channel 24 by modifying its token 26 to indicate the availability of the channel 24.
Since each token 26 is shared among all the stations 14 in the network 10 and signifies only availability of transmission slot, it is completely insensitive to the types of services and does not provide for any bandwidth fairness issue. For instance, a station 14 requesting service for a best effort type application would receive the same treatment as a station 14 requesting service for a time-sensitive application. Furthermore, the tokens 26 do not provide for an associated expiration of service, so a greedy application can consume substantial portion of the bandwidth. Finally, the round-trip time of a token, stemming from the fact that a station has to wait up to a full round-trip to sense an empty token 26, can pose significant delays, especially as the network diameter increases. Accordingly, the strains on the electronic buffers of the stations 14 can become pronounced.
Therefore, a need has arisen for a more efficient metropolitan access network and associated protocol that accounts for quality of service issues.